Keep texture shadeless, but recieve shadows?
I don't know if there is a way to do exactly what you're talking about, but I have another idea.

I would duplicate the clean plate and move it ever so lightly closer to the camera (as little as possible). Then change its material to a transparent shadow-catcher. That way, you can see the plate through/behind it, but the letters can still cast shadows on it.

I'd love to see how well it works ;-)

I hope that helps!